Do you think that it would work well with the pedals in front of, or behind, the hub? That would solve the pedal strike issue, but avoid the extra height that would come from having them above (which would be less desirable in the event of UPDs)

I think having the pedals a bit higher is actually better in the event of a UPD. I find myself a bit too close to the ground when riding a geared 29" -- I quite like the pedal height of a 36". If you are too close to the ground then I think it might be harder to get your feet ready for landing, thus making the UPD worse.

So, I think I'd be happier with the Huni-Rex if the pedals were above. It would make the frame larger and less portable, but it would make for a very strong frame, and a comfortable height. It would still be missing the best feature of Red Menace which is the ability to change the gear ratios.

But, I'm not sure about the pedal height. I try to avoid UPDs, not do scientific research on them.

I think having the pedals a bit higher is actually better in the event of a UPD. I find myself a bit too close to the ground when riding a geared 29" -- I quite like the pedal height of a 36". If you are too close to the ground then I think it might be harder to get your feet ready for landing, thus making the UPD worse.

So, I think I'd be happier with the Huni-Rex if the pedals were above. It would make the frame larger and less portable, but it would make for a very strong frame, and a comfortable height. It would still be missing the best feature of Red Menace which is the ability to change the gear ratios.

But, I'm not sure about the pedal height. I try to avoid UPDs, not do scientific research on them.

I'd prefer my pedals lower in the event of a UPD- obviously not so low that pedal strikes are an issue, as they are with the current location of the pedals beneath the axle. So I'd probably want them level with the axle which would require them either in front, or behind, the axle- not sure if that would create difficulties when riding.

__________________
"You can't outrun Death forever.
But you can make the Bastard work for it."

Trying to stand up with the pedals in front or behind the axle would certainly create "difficulties".

Would it? Why?

I'm not saying it definitly wouldn't, but I don't think it's something you can just state as a fact- a bit of reasoning is necessary to back up your opinion.

And, I am an experienced unicyclist- my instinctive feelings on it are that having the pedals a few inches front/back of the axle wouldn't create any instability that couldn't easily be compensated for by a slight lean.

__________________
"You can't outrun Death forever.
But you can make the Bastard work for it."

I'm not saying it definitly wouldn't, but I don't think it's something you can just state as a fact- a bit of reasoning is necessary to back up your opinion.

And, I am an experienced unicyclist- my instinctive feelings on it are that having the pedals a few inches front/back of the axle wouldn't create any instability that couldn't easily be compensated for by a slight lean.

Seems to me it would. There's an inherent difference between a verticle line down from your CG through the pedals and axle, vs not. I'm no physicist so can't really expain it well, but in my experience gravity works, and it works straight down. If you stood up on pedals that are not in that straight verticle line, it would tend to rotate the pedals down below the axle until gravity had acquired equilibrium again. You might be able to keep things upright by holding the handle, but there would still be an issue.

Internal hub gears: expensive, quite heavy, and has to be precisely made. Can be shiftable. The "posh engineering" solution.

Chain drive like a fixie. Simple, robust, readily available parts. Can't easily be changed. Needs the centre of rotation of the cranks to be above the hub for best results. (Above still affects balance and mounting.) Below the hub causes pedal strike problems. In front of or behind the hub makes a different approach to riding necessary.

Two chains in series, with a jackshaft, on each side. Slightly clumsy and heavy, but retains the common centre of rotation for the cranks and wheel.

There must be other options.

Kevlar toothed belts are used on some powerful motorbikes and a small number of single speed or hub-geared bikes. I think a Kevlar toothed belt can be set up with less slack than a chain. It may therefore be lighter and more responsive.

A drive shaft. Cars and some motorbikes use shaft drive. Outboard motors use shaft drive. At least one manufacturer makes a shaft drive for a bicycle. A shaft drive may be less efficient than a chain, but may also be more compact, cleaner (possibly enclosed within the fork legs?) and may allow for changeable gears.

A jack shaft arangement with the cogs meshing directly rather than a chain connection. Saves on the weight and complexity of the chain, but I imagine there would be big friction losses.

On a unicycle, the force of your weight needs to be directly through the contact point on the ground, or you fall down.

Your center of mass needs to be above the contact point to be stable -- that much is clear. If the pedals are six inches in front of the axle then they will be roughly six inches in front of the contact point, along with the mass of your legs. You then need to be leaning backwards to keep your center of mass above the contact point. The top of your body would need to be behind the vertical line above the contact point just to compensate for your legs. That feels like it would be a lot of lean.

It's worth noting that this lean isn't just needed when trying to stand up. Unless you are accelerating you need your center of mass above the contact point, so you would be constantly leaning backwards (or forwards if the pedals were mounted behind). I don't think it would be hard to ride, but I wouldn't be surprised if it would get uncomfortable after a while.

Riding one would be just like riding a wheelie on a bike, and as long as the seat were a little back of center riding seated would be easy enough. Artistic cycling folks do this all the time.

Standing would be like riding a standing wheelie on a bike, or rear wheel hopping on a trials bike. If you've tried either of these, you would know that you have to pull back really hard on the handlebars to hold your position. If you let go, the front end would slam down instantly. Doing this on a unicycle with nothing to grab onto but the front of the seat would be a huge pain; even with the long leverage a bike frame gives, it's pretty fatiguing on my trials bike. A longer handlebar would work better, but current designs don't seem strong enough to me.

The other uni layout with pedals-behind-the-wheel would work the other way around, with the seat forward, and you having to push down hard on something in front if you stood up. Which sounds horrible.

Riding one would be just like riding a wheelie on a bike, and as long as the seat were a little back of center riding seated would be easy enough. Artistic cycling folks do this all the time.

Good analogy. The unicycle with pedals in front would be less dramatic than riding a wheelie on a bike because the pedals would be about six inches in front instead of ~20-30 inches, but the same principal should apply. That is, it is totally doable, but not ideal.

A drive shaft. Cars and some motorbikes use shaft drive. Outboard motors use shaft drive. At least one manufacturer makes a shaft drive for a bicycle. A shaft drive may be less efficient than a chain, but may also be more compact, cleaner (possibly enclosed within the fork legs?) and may allow for changeable gears.

I've wondered if a shaft drive would have less slop than a chain drive or the Shlumpf. My intuition is that it would be an improvement.

Chain drive like a fixie. Simple, robust, readily available parts. Can't easily be changed. Needs the centre of rotation of the cranks to be above the hub for best results.

No it doesn't. It does on the Huni-Rex, but the point is to avoid pedal strikes. Just using a 29" wheel would cure the Huni-Rex, I think. Cranks can also be in front, rear or at some odd angle to keep them from being too low.

However, putting them anywhere other than above/below the wheel axle will add frame weight.

Quote:

Originally Posted by MrImpossible

On a unicycle, the force of your weight needs to be directly through the contact point on the ground, or you fall down.

So if the pedals are ahead of the contact point, you can't put your weight on them unless you somehow lean way back to compensate, pulling against a handle or something.

It would take some getting used to, but I don't think it's as major a problem as you seem to think. Kind of like learning to leverage your unicycle with an extended handle, after years of riding with just a seat handle. It's different, but you get used to it.

For out-of-saddle climbing, you would probably just need a handle that stays within reach when you're in that position. It may also feel a little funny, with the pedals not being in vertical alignment with the wheel axle, but that would also just take some practice to get used to. Like riding in a gear other than 1:1 after many years of only riding 1:1.

I originally suggested putting the pedals out front in the interest of improving the riding position (for seated cruising). The pedals are always ahead of the seat on a bicycle, and bicycle seats are still better for really long rides (on bikes).

Quote:

Originally Posted by MrImpossible

Standing would be like riding a standing wheelie on a bike, or rear wheel hopping on a trials bike. If you've tried either of these, you would know that you have to pull back really hard on the handlebars to hold your position.

The amount of pull depends on the (vertical) offset between the wheel axle and pedal axle. The less offset, the less work involved. Like riding an artistic bike on the rear wheel, with one foot pedaling and the other on a rear foot peg. As you pull the handlebars up toward you, the amount of force required goes way down as the pedals get more above the axle. When you get it all the way up, it's not difficult at all.

Quote:

Originally Posted by MrImpossible

Doing this on a unicycle with nothing to grab onto but the front of the seat would be a huge pain; even with the long leverage a bike frame gives, it's pretty fatiguing on my trials bike.

MrImpossible, I think your handle (name) is making you a naysayer. A KH-type seat handle would be more than sufficient. The handle would be pulling back, but not that much. In your trials bike example, your pedals are probably still quite a ways ahead of the rear axle at your highest position. The unicycle we're envisioning would not have such an offset.

Besides, the purpose of a geared unicycle would mostly be for cruising, which is mostly in a straight line and on non-technical terrain if you can't shift it. Mostly you would only be out of the saddle for uphill climbing.

I said "for best results" and then considered the disadvantages of the other obvious positions. In particular, that putting the centre of rotation below the hub would increase the risk of pedal strikes.

Centre of rotation of the cranks above the hub: unicycle is taller. Mounting, idling and safety on the road may be compromised. However, the rider is in approximately the "familiar" position of a conventional unicycle.

Centre of rotation of the cranks below the hub: risk of pedal strikes. Lower centre of gravity may make the uni more twitchy.

Centre of rotation of the cranks in front of the hub. Rideable (like a "unibike") but a totally different technique is required. For example, standing on the pedals for hills is an issue. May be good for cruising.

Centre of rotation of the cranks behind the hub would require a very weird seating and riding position indeed.